Unraveling the Mystery of Galicia's Ancient Shad Populations
Exploring the biology and ecology of Allis shad and Twaite shad in Galician waters, their conservation challenges, and scientific discoveries that could ensure their survival.
Imagine a typical morning at the fish market in A Guarda, Galicia. Fishermen unload their catch—silvery hake, flatfish, and sea bass spill across the auction floor. But among these familiar species lie other, less-known fish: slender, silvery migrants whose life story remains largely untold. These are the shad, ancient travelers whose marine mysteries have begun to unravel only through their unintended capture as bycatch 1 3 .
Alosa alosa - The larger of the two species, with more gill rakers and a preference for plankton.
Alosa fallax - Smaller with fewer gill rakers and more pronounced fish-eating behavior.
For decades, Allis shad (Alosa alosa) and Twaite shad (Alosa fallax) have arrived quietly at Galician fish markets, mistaken for one another or simply labeled as unspecified "Alosa." These anadromous species spend most of their lives at sea but return to rivers to spawn, much like salmon. Yet unlike their famous relative, shad remain in scientific shadow, their marine lives poorly understood despite their ecological importance 1 .
To understand the shad's ecological importance, we must first understand their extraordinary life history. Both Allis and Twaite shad belong to the Clupeidae family, making them relatives of herring and sardines. But unlike their purely marine cousins, shad are anadromous, meaning they spend most of their life in saltwater but migrate up freshwater rivers to spawn 1 .
Adult shad return to their natal rivers to spawn after several years at sea.
After hatching, juvenile shad remain in freshwater for several months before migrating to the sea.
Shad spend most of their lives in coastal marine waters, feeding and maturing.
Mature shad return to freshwater rivers to complete the life cycle.
This life strategy connects marine and freshwater ecosystems in a vital ecological bridge. Shad transport nutrients from the ocean to rivers and back again, supporting food webs across ecosystem boundaries. After hatching in rivers, juvenile shad migrate to the sea, where they will spend several years maturing before returning to their natal rivers to reproduce .
Telling Allis and Twaite shad apart has long challenged fishermen and scientists alike. The two species are congeneric (closely related) and can even hybridize, creating individuals that blur the already fine lines between species.
Traditional identification relying on external features often fails, requiring scientists to look deeper—literally—at characteristics like gill rakers for positive identification 1 .
To address critical knowledge gaps about shad populations, researchers designed a comprehensive study focusing on bycatch individuals landed at Galician fish markets.
Regular visits to three key fish markets—A Guarda, Malpica, and A Coruña—to acquire Alosa specimens (N=345).
Examining first-sale fishing statistics from 1997 to 2020 to understand long-term catch trends.
Gathering information about capture circumstances directly from fishermen.
The key to distinguishing the species lies in their gill rakers—comb-like structures that help fish filter feed.
The meticulous examination of Galician bycatch yielded unexpected insights into the region's shad populations, challenging previous assumptions and revealing complex ecological dynamics.
Perhaps the most striking finding concerned the true identity of the fish being landed. Historical data had shown regular Allis shad catches from 1997-2020 totaling 23,956 kg, with Twaite shad appearing only in occasional pulses—5,950 kg in 1998 and 7,320 kg in 2018 3 . However, when researchers examined the 345 bycatch specimens through gill raker analysis, they discovered a different story:
Stomach content analysis revealed a common trophic spectrum across all three taxa, including fish, plankton, benthos, and cephalopods. However, important differences emerged in feeding preferences:
| Taxon | Primary Feeding Preference | Secondary Food Sources |
|---|---|---|
| Alosa alosa | Plankton and small invertebrates | Fish and cephalopods |
| Alosa fallax | Fish (ichthyophagous) | Plankton, benthos, and cephalopods |
| Hybrids | Mixed diet | Intermediate between parent species |
Twaite shad showed more pronounced ichthyophagous behavior than both Allis shad and hybrids, suggesting niche partitioning that may reduce competition between these closely related species 1 .
| Research Tool | Primary Function | Application in Shad Research |
|---|---|---|
| Gill Raker Analysis | Species identification | Distinguishes between A. alosa, A. fallax, and their hybrids based on gill raker count 1 |
| Stomach Content Analysis | Trophic ecology determination | Identifies dietary preferences and niche partitioning between species 1 |
| Market Sampling | Bycatch assessment | Documents species composition and fishing pressure on populations 3 |
| Age Structure Analysis | Population demography | Uses scales or otoliths (ear stones) to determine age distribution and mortality 1 |
| Spatial Mapping | Habitat identification | Plots capture locations to identify critical habitats and propose protected areas 1 |
The Galician bycatch study comes at a pivotal moment for EU fisheries management. Recent reports indicate that while the overall sustainability of EU fisheries continues to improve, with more fish stocks being fished at sustainable levels, key species continue to struggle 4 .
The study proposes two new protected areas to enhance shad conservation while fulfilling EU obligations toward these species 1 .
The widespread misidentification of shad species underscores the need for better identification protocols and monitoring 3 .
The story of Allis and Twaite shad in Galician waters embodies the complex challenge of managing and protecting migratory species in the modern ocean. They connect marine and freshwater ecosystems, cross national boundaries, and defy easy classification—both literally in terms of species identification and figuratively in their conservation status.
As the European Commission works toward greater fisheries sustainability 4 , research like the Galician bycatch study provides the essential scientific foundation for effective management. By transforming unidentified bycatch into detailed biological data, scientists have given voice to these silent migrants, revealing not just two species but a more complex ecological picture including hybrids and shared habitats.
The path forward requires integrating this new knowledge with conservation action—whether through expanded protected areas, improved fishing gear selectivity, or enhanced monitoring. As we better understand these ancient travelers, we take crucial steps toward ensuring that future generations will still witness the silver flash of shad in Galicia's coastal waters, not just as bycatch but as thriving members of marine ecosystems.